Static caravan emergency backup — where do I even start with battery sizing?

Right, so I've finally convinced myself to sort out a proper backup system for the static. We're up in the Scottish Borders and the grid here can be properly unreliable in winter — last February we lost power for about 14 hours during a storm and it was grim. Fridge full of food, no heating controls, nothing. Never again.

The van runs on mains for everything day-to-day. I'm not trying to go full off-grid, just keep the essentials alive: fridge (about 100W running), the gas boiler's control board and pump (~80W), a few lights, and phone charging. Rough guess puts me at maybe 400–500Wh actual draw over a bad night, but I honestly don't know if I'm calculating that right.

I've been looking at a Victron MultiPlus-II as the inverter/charger because it does that seamless transfer thing, and either a couple of Fogstar Drift 100Ah 12V lithium batteries or stepping up to 24V. What I can't work out is whether 200Ah at 12V (so ~2.4kWh usable with LiFePO4) is actually enough headroom, or whether I'm undersizing it and will regret it. Has anyone built something similar for a static or holiday home backup specifically?

DefenderConvert | Posts: 847

@Sam1975 Classic Borders situation — I know it well! Before anyone can give you sensible sizing advice, you really need to answer three questions first:

1. What are you actually protecting? Heating controls, lighting, fridge, the lot?
2. How long do outages typically last? A few hours or multi-day events?
3. Do you have any solar on the roof already?

A static caravan is actually a lovely candidate for a decent LiFePO4 setup because the loads are usually quite manageable. Your biggest enemy in Scottish winters isn't capacity — it's charging. Without solar or a generator feeding the batteries, you're limited to what you stored before the outage hit.

Start by listing every appliance with its wattage and rough daily hours of use. That'll give us something concrete to work with rather than just guessing numbers at you.

Cerbo_Geek | Posts: 1,243

@Sam1975 Key question nobody's asked yet: what's your critical load versus your comfort load?

For a static, I'd separate them ruthlessly:

• Critical: fridge, heating controls, lighting, phone charging — probably 200-400Wh/day
• Comfort: TV, kettle, microwave — easily 10x that

Size your battery bank for critical loads × 3 days autonomy minimum (Scottish winters mean extended low-irradiance periods — 3 days of usable solar is optimistic in February).

Running a Victron Multiplus-II on my static with 15kWh of Fogstar Drift LiFePO4. For pure backup without daily solar cycling, LiFePO4 is overkill budget-wise — decent AGM from Varta or Platinum might actually serve you better short-term.

What's your heating system — gas with electronic ignition, or oil? That's often the silent killer for backup sizing that catches people out.

Lefty31 | Posts: 312

Got a cabin setup myself — lesson I learned the hard way is to actually measure your loads rather than guess. Grabbed a cheap plug-in energy monitor off Amazon for a tenner, ran it across everything for a week. Eye-opening stuff.

Also worth thinking about days of autonomy not just capacity. Scottish winters mean you might get sod all solar for 3-4 days running. My Fogstar cells are rated for that kind of buffer and they've been solid.

Don't skimp on the BMS side either — Victron kit talks to everything nicely if you go that route later.

What's your heating situation? That'll likely dominate your numbers more than anything else @Sam1975

LisaHunt88 | Posts: 156

@Sam1975 Static caravan owner here so very much my wheelhouse.

One thing not mentioned yet — static caravans lose heat fast, so if heating is on your critical list, factor in that your inverter load will be far higher in the Borders winter than any online calculator suggests.

I run a Victron MultiPlus-II paired with Fogstar Drift lithium cells and honestly the Victron's built-in transfer switch is brilliant for seamless grid-to-battery switchover. No flicker, fridge doesn't even notice.

Also worth checking whether your caravan has a consumer unit rated for an inverter feed — mine needed upgrading before any of this made sense, cost about £180 but saved a headache later.

What size pitch are you on? Single unit or twin-unit static? Makes a big difference to realistic consumption figures.

ForestJenny | Posts: 847

@Sam1975 Scottish Borders winters are brutal on a system — I learned this the slow way with my own static before I upgraded properly.

One thing worth factoring in that nobody's touched on yet: temperature derating. Lithium batteries lose usable capacity in the cold, and if your battery bank is sitting in an unheated compartment under the van, you'll have noticeably less capacity than the spec sheet promises come January.

Fogstar Drift cells handle the cold reasonably well in my experience, but worth checking the manufacturer's low-temperature discharge figures before committing to a size.

Also — Scottish Borders means limited winter solar yield. Don't size your system assuming solar will bail you out between November and February. Shore power charging or a small genny as backup deserves a spot in your planning from the start.